Funds are requested for gas chromatograph\combustion/isotope ratio mass spectrometer (GC/COMB/MS). This instrument will dramatically increase our capabilities to do in vivo human metabolic studies. Currently, we can perform studies on feces, urine, and breath to measure whole body absorption and utilization of macronutrients. This instrument will permit us to do studies of transport and metabolism of macronutrients using blood sampling. The GC/COMB/MS offers the advantages of small sample size and separation of conventional GC/MS plus the high precision of conventional isotope ratio mass spectrometer for measurement of excess 13C in highly diluted samples. There are five funded major users. The measurement of arterial plaque remodeling, the study of fetal lipogenesis in the baboon model, the study of postprandial fat transport in diabetes mellitus, the determination of alphalipoprotein kinetics, and investigations into the effect of diet and exercise on energy balance in the treatment of human obesity. In addition, there are two minor users who will investigate the origins of urinary acylcarnitines. The acquisition of the GC/COMB/MS would provide heretofore unavailable capabilities at the University of Chicago for 13C analysis of small samples and open new avenues of investigation.